Are you worried that you or someone close to you might be becoming too fat or obese? What health problems might this lead to? What are the symptoms?

What is Obesity? 

Obesity means being extremely overweight and carrying too much body fat i.e. with a Body Mass Index (BMI calculator) of over 30. BMI is calculated by comparing your weight and your height.

A BMI of between 20 and 25 is considered healthy. BMIs between 25 and 30 are considered overweight and BMIs between 30 and 39.9 obese. This is a useful rough guide, although the BMI calculation can’t distinguish between excessive fat or simply being muscular.  

You can also use this NHS chart to compare your weight against your height. Simply step on the scales and check how you compare.  

How is obesity caused?  

The main cause of obesity is often described as eating too much while not getting enough exercise. regularly consuming more calories than we burn off.  As we report elsewhere on the website, exercise has many health benefits and can help manage weight. 

However, research now suggests that it is how much food we eat and what type of food that is primarily causing obesity - and that this in turn reflects changes in the global food system, including food now being readily available at a much lower time cost.  

Food high in sugar and fat is a particular risk. For example sugar sweetened drinks are implicated in obesity among both children and adults. They add calories without making us feel full, so we tend to eat as much as usual without realising that the soft drinks have been adding extra 'hidden' calories. 

Eating whenever food was available probably made evolutionary sense. For thousands of years our human ancestors could never be sure where the next meal was coming from, so the human body evolved to store fat to help people survive when food was scarce. 

However, today food is readily available from many sources, from fast food outlets to supermarkets. Meanwhile, changes in technology, like motor vehicles, mean we are now less physically active. These two factors increase the risk of storing more fat and becoming obese.  

Research suggests that what type of food we eat also matters. People who are obese haven't usually been binge-eating vegetables, fruit and wholegrains. They are more likely to have been eating convenience foods high in sugar, salt, fat and refined carbohydrates. For example one review of 50 studies estimated that increasing fibre intake by 14 grams per day was associated with a 2 kilogram weight loss over about a 4 month period - and fibre is more common in vegetables, fruit and wholegrains.

Studies with families, twins and adoption cases suggest an association between genetics and obesity i.e. that some people may inherit 'fat genes.' If this proves to be the case it would suggest some people may face more of a challenge than others. However, large scale obesity is a recent trend historically and our genes don't change that quickly. This suggests that if there is a 'fat gene' it was only triggered relatively recently as food became so abundantly available.   

Is obesity potentially life-threatening? 

Obesity is associated with a higher risk of chronic disease and death among non-smokers without previous disease. One estimate is that if you’re over 40 and obese you’ll die up to 10 years sooner than you would if you had been a normal weight. Here are some of the increased health risks people face if they are obese: 

Type 2 diabetes 

 Type 2 diabetes can lead to heart disease and stroke, vision loss and blindness, miscarriage and stillbirth - and can even result in having toes, feet or lower legs amputated. It is a condition where your body stops responding to insulin, a hormone than regulates blood sugar levels. Excess body fat interferes with the body’s ability to effectively use insulin, increasing the risk of developing Type 2 diabetes. Diabetes UK report that obesity is believed to account for 80-85% of the risk of developing Type 2 diabetes 


 According to Cancer Research UK, obesity is the second largest preventable risk factor for cancer in the UK, after smoking, with more than 1 in 20 cancer cases caused by being overweight. Breast cancer and bowel cancers are the most common types associated with obesity. 

Hypertension and Cardiovascular diseases

Having excess weight and a high salt diet can result in high blood pressure. An abnormally high blood pressure (140/90mmHg or higher) is known as hypertension and it can strain your blood vessels and organs. Hypertension is a major risk factor for developing heart disease, heart failure and stroke. The more body fat you have, the harder your heart has to work to pump blood around the body, eventually straining the heart.

Other health risks include infertility, osteoarthritis, chronic back pain and depression. 


  • A BMI of 30 – 39.9 is considered obese.
  • The main cause of obesity is eating to much of the wrong kind of food
  • Not doing enough physical activity is another factor
  • Being obese could knock up to 10 years off your life – it increases your risk of heart attack, stroke, diabetes and cancer.

To find out how to prevent obesity, read our guide.

Reviewed and updated by Nicole Musuwo, November 2018. Next review date October 2022.